Introduction
In recent years, the intense focus has been given to finding sustainable
energy solutions that are cost-effective and have a minimal
environmental impact, and, and are abundant in nature. Several energy
harvesting techniques, such as
piezoelectricity,[1]triboelectricity,[2]thermoelectricity,[3] have been thoroughly
explored to convert mechanical, thermal, and renewable energy into
electricity. However, each approach has barriers, such as extensive
material costs, complicated preparation procedures, limited resources,
and inadequate electrical output, which limit their practical use. It is
crucial to maximize the energy extraction from a pure source for
long-term feasibility. Because of its abundance (71% of the Earth’s
surface) and significant intrinsic renewable energy potential, water has
gained increasing interest for its use in prominent power generation.
While traditional approaches like thermal energy
extraction,[4] or
hydroelectricity,[5,6] generation has been widely
exploited, hydrovoltaic method has recently emerged as a viable method
for harnessing energy.[7-10] This unconventional
hydrovoltaic system can harness electricity through the direct
interaction between functional materials and various forms of water,
such as water droplets,[11] flow of
water,[12,13] wave,[14,15]evaporation,[16-19] and
moistures.[20] Evaporation-based energy harvesting
from water droplets has recently gained attention in relation to the
hydro-voltaic effect.[7,8]Utilizing the interactions between the water droplet and solid material
interfaces, this method can generate electricity by leveraging physical
phenomena such as streaming potential,[21] ionic
motion coupling,[22]triboelectrification.[23] Since efficient energy
harvesting from sustainable sources is crucial, evaporation-driven
electrical generators (EEGs) are a potential solution for the progress
of renewable energy systems. This EEG process is more spontaneous and
can utilize maximum energy from water.[17] Several
novel nanomaterials have garnered significant attention in the realm of
hydrovoltaic energy harvesting, which yields electrical potential at the
interface between water and a polarizable
substance.[24] Despite the promising
characteristics of the novel nanomaterials, such as high conductivity
and surface area, their feasibility is limited by their high costs,
limited availability, resistance to water flow, and labor-intensive
preparation processes. Over the years, naturally available bio-based
materials having porous structures were overlooked. Recently,
microchannels of different natural wood structures have been exploited
for hydrovoltaic energy harvesting.[25-27]However, most of the existing organic material-based WEGs exhibit lower
power density, hindering their suitability for practical device
application. Moreover, the the device size would be substantially larger
because of the required shape of the wood[27] and
the required amount of water is considerably higher. On top of that,
some forms of pretreatments are required to improve the power
density.[26]In this study, we investigate a new approach- evaporation-driven Water
induced Electric generators (WEG) by utilizing natural nutshells, which
possess a porous micro/nanochannel structure[28]and sufficient polar functionalities.[29] This
natural anisotropic 3D micro/nano-channel architecture of nutshells
(NSs) facilitates efficient water and nutrient
transport.[30] For the first time, the
micro/nanochannels within the NSs (Almond Shell - AS, Filbert Shell -
FS, Pecan Shell - PS, Walnut Shell - WS) have been successfully utilized
to harness hydrovoltaic energy through evaporation by utilizing the
concepts of streaming potential as illustrated in Figure 1 . A
simple one-step preparation process can harvest above 600 mV with a
power density of 5.95 µW·cm−2. The efficiency of this
device is affected by critical factors such as density, porosity,
contact area, surface charge, and hydrophilicity. By applying some
chemical treatments, the surface area and porosity of the nutshells
(NSs) have been augmented. The subsequent highly porous shells exhibit
an impressive output potential of 1.21 V and a maximum current density
of 347.2 µA·cm−2 under the synergistic effect of
streaming potential and chemical reactions. These innovative and
functional NS-powered energy generators are cost efficient and
environmentally friendly, rendering them suitable for powering small
electronic devices. Moreover, this innovative technology presents a
novel approach to green energy solutions and broadens the range of
materials that can be potentially utilized for Water induced Electric
Generators (WEGs).